The evolutionary transformation of the upper valve (UV) to a probable particle filter system in some hippuritid genera belongs to the most drastic changes in pelycipod shell structures. The basic architecture of this product is well known since long times. Again discussed is the functional role of the canal system. The bottom of the canals is very smooth.
Trang 1The Morphology and Function of the Upper Valve of
Vaccinites vesiculosus (Woodward)
DIETRICH SCHUMANN
Institut für Angewandte Geowissenschaften der Technischen Universität Darmstadt, Schnittspahnstrasse 9,
D-64278 Darmstad, Germany (E-mail: schumann_dw@web.de)
Received 1 April 2009; revised typescript received 21 July 2009; accepted 6 October 2009
Abstract:The evolutionary transformation of the upper valve (UV) to a probable particle filter system in some hippuritid genera belongs to the most drastic changes in pelycipod shell structures The basic architecture of this product is well known since long times Again discussed is the functional role of the canal system The bottom of the
canals is very smooth The canal system of Vaccinites vesiculosus is completely roofed by a particular graceful sieve A
water exchange between the canals did not exist The uppermost part of the upper valve (the sieve) most probably was
an endoskeleton The upper valve was not accreted with the lower valve Premortal damages of the canals and sieves could be repaired It is dicussed whether the oscules E and S really were places of exhalent currents Early ontogenetic growth stages of the canal system are unknown The pattern of the canal arrangement always is an individual one.
Key Words:palaeobiology, hippuritids, Vaccinites, feeding current, canal pattern, sieve structure, growth
Vaccinites vesiculosus (Woodward) Üst Kavkısının Morfolojisi ve Fonksiyonu
Özet:Üst kavkının (UV) evrimsel dönüşümü, pelesipod kavkı yapılarındaki köklü değişikliklerin birçoğuyla ilgili olan bazı hippuritid cinslerindeki parçalı filtre sistemiyle olasıdır Bu ürünün temel mimarı uzun zamandanberi çok iyi
bilinmektedir Kanal sisteminin fonksiyonel rolü birkez daha tartışılmıştır Kanalların dipleri oldukça düzdür Vaccinites vesiculosus’un kanal sistemi tümüyle özel bir elekle örtülmüştür Kanallar arasında su değişimi olmamıştı Üst kavkının
en üst kısmı (elek) büyük bir olasılıkla endoskeletondu Üst kavkı, alt kavkıyla eklenmemişti Kanal ve eleklerin ölüm öncesi hasarları onarılabilmişti E ve S boşluklarının gerçekten dış akıntılarının yeri olduğu tartışmalıdır Kanal sisteminin erken ontojenetik büyüme evreleri bilinmemektedir Kanal düzeninin modeli daima bireyseldir.
Anahtar Sözcükler:paleobiyoloji, hippuritids, Vaccinites, beslenme akımı, kanal modeli, elek yapısı, büyüme
Introduction
In the Early Campanian Samhan Formation of
Central Oman (for localities and detailed lithological
sections and stratigraphy see Platel et al 1994 and
Schumann 1995) large rudist associations with an
extension of hundreds of square kilometres exist
The most common species within these
autochthonous communities is Vaccinites vesiculosus.
The last comprehensive studies of Vaccinites were
done by Laviano et al (1992), Simonpiétri (1993)
and Simonpiétri et al (1998) These investigations
dealt mainly with stratigraphical, phylogenetical,
palaeogeographical and biometric-statistical
questions The particular structure and function of
the upper valve was not described or discussed Among the countless pieces from the Saiwan-localities are many specimens with best preserved upper valves (forthcoming called UV) The surfaces
of the UV (sieves) and the underlying canal system
of the UV were excellent prepared in all stages by the natural sand blasting of the desert (Plate 2, Figures 3-5) This enables unusual observations and leads to the questions discussed here
For the principal function of these particular hippuritid UVs two hypothesises were made First, according to Douvillé (1897) and Skelton (1976) the trapped particles (seize-controlled by the sieve) were leaded into the underlying radial canals and then
Trang 2inside passing through the marginal openings which
are distributed around the whole commissure of the
UV Second, Pons et al (1994) and Seilacher (1998)
on the other hand were more believing in a
photosymbiotic function Pons et al described the
probable existence of ‘expansiones tentaculares’ and
‘asociación con zooxantellas’ in connection with the
left mantle of Hippurites cornucopiae Seilacher had
similar ideas with ‘pallial diverticles, retractable
papillae’ and ‘algal gardens’ for photosymbiosis For
the older history see Dechaseaux & Coogan (1969)!
Which hypothesis is more probable critically
depends on nature and function of certain
morphological features, such as the areas of the three
infoldings in the UV, the areas of the ligamental ridge
and the oscules S and E
Materials, Locality
180 complete and well-preserved specimens, 22
isolated upper valves, mostly incomplete Large cliffs
8 km south of the abundoned air strip Saiwan
(Central Oman); GPS-coordinates: N 20° 3’ 32,6’’ E
057° 36’ 49,7’’
Results
The Omanian specimens, all belonging to V.
vesiculosus, show a distinct individual variability of
the canal arrangement (Plate 1, Figure 2a; Plate 2,
Figures 1 & 2) Only the width of the canals and the
socle distance (denticular base of the sieve on the
canal ridges) are relatively identical in all pieces of V.
vesiculosus.
The principal arrangement of the canal system is
radial The arrangement always comes in spacial
conflict in the areas of the three infoldings L, E and
S There the canals are running individual ways onto
or around the infoldings, and often collide with
neighbouring canals (Plate 1, Figure 2a; Plate 2,
Figures 1 & 2) At the end of each canal V vesiculosus
shows a main ‘gully-hole’ and in addition sometimes
also some smaller openings around the whole
margin The floors of the canals are very smooth and
there are no vascular impressions at all (REM
control) The pores are not circular, they are
sharply pointed The surface of the sieve is densely equipped with short spines (Plate 1, Figure 2a; Plate
2, Figure 3) Some specimens show that also the areas
of the oscules were roofed by a sieve And around pillar E and S in the lower part of the UV always a ring of flat-oval vertical canals is visible, completely roofed by the sieve (Plate 1, Figures 1 & 2a, b) Already Vogel (1960) has observed such additional
canals in his sections of Hippurites socialis and
believed, that they probably ‘washed’ the pillars
The uppermost structure of the UV of V.
vesiculosus is a rather evolved one The sieve is very
thin, the diameter of the pores is 0.6 mm in average, often smaller Somewhat bigger pores are subdivided
by fine secondary net structures Some areas show bushwoodlike thickenings On the outermost margin the sieve has a very graceful finger-shaped zone (Plate 2, Figures 4 & 8)
Discussion
Douvillé (1897) suggested that the pillars of the lower valve correspond to inhalant (E) and exhalent (S) currents Klinghardt (1930, 1931) and Wiontzek (1937) reported cavities within the pillars and were convinced that the pillars were siphonal tubes Milovanovic (1933) and Kühn (1937) recognized these cavities as diagenetic structures, the pillars are massive structures Vogel (1960) did careful
sectioning in the area of the pillars of Hippurites
socialis He reported that the oscules were always
open and therefore probably were places for sense organs or accessory gills The last comprehensive palaeobiological publication dealing with these questions is the admirable analysis of Skelton (1976)
He agreed with Vogel (1960) that the oscules were always open, but refused to believe the existence of sense organs or accessory gills in the oscules area Pons (in Cestari & Sartorio 1995) showed the best published section with the structure of the UV of the
Santonian speciesVaccinites oppeli The uppermost part of the UV (the sieve) of V vesiculosus is
definitely less compact in comparison with the
illustrated specimen of V oppeli.
Also the canals are described since more than a century, but I found a graphic presentation of a
Trang 3(remarkably an Omanian specimen of V vesiculosus
from Saiwan) Even an ontogenetic or phylogenetic
study of any canal system does not exist Zapfe
(1937), Vogel (1960), Skelton & Gili (1991) and Götz
(2003) described early ontogenetic stages of
hippuritids without mentioning any early stages of a
canal system The classic literature shows that the
architecture of the canal system seems to be rather
different within the taxonomic groups
Skelton (1976) in his most interesting analysis
interpreted the oscules E and S as places of exhalent
currents But, if covered by sieves, could they than
have served for exhalent currents? A sieve normally
is controlling the entrance of particles Outgoing
particles (faecal/pseudofaecal) often are bigger as the
trapped ones Is it eventually not more likely that the
whole surface of the UV was used for the particle
influx with the aid of ciliated epithelia? The exhalent
currents could have been located on another place of
the margin As many modern pelycipods and
brachiopods show a distinct morphology respective
within the shell margin is not necessary for this
function
Because of the new observations I have doubts
that the described system was exclusively or mainly
used for photosymbiosis, with tentacles in the canals
and/or retractable papillae I believe more in a
perfect water circulation system The spiny surface of
the sieve probably helped to reduce the flow velocity
and therewith to trap useful particles This does not
mean that not any other parts of the mantle tissue
with ‘algal gardens’ could have existed, a function
which probably was of high significance in some
rudist groups
Conclusions
Generally pelycipods have an additive grown
exoskeleton produced by a generative zone But
many inner parts do not grow simply additive A
bivalve tooth e.g can grow only by a permanent
secretion and resorption of shell material,
nevertheless it is classified as an exoskeleton I
presume that the sieves of the UV of V vesiculosus
were growing in an analogue way The lower part of
the UV grew principally as an exoskeleton, while the upper part (finally a sieve) evolved to an
endoskeleton Pons et al and Seilacher also were
convinced of a tissue-embedded UV Considering
the remarkable fine structure of the vesiculosus-sieve
(Plate 1, Figures 1, 2b; Plate 2, Figures 3, 4 & 8), especially the very graceful finger-shaped marginal structures, it could be an embedded endoskeleton only Presumably at least the whole uppermost part
of the UV was embedded by a thin ciliated epithelial tissue Thus it could grow and evolve with all the possibilities of an allometric growth
The UV of V vesiculosus was never accreted with
the lower valve There was no premortal settlement
of any epibionts at all on the surface of the sieves Between hundreds of specimens of my collection only one sieve was damaged during life and roughly repaired (Plate 2, Figure 2)
I have no idea why a functional important
structure as the canalization of Vaccinites has such an
individual arrangement It is like a zebra: We see a significant pattern, but it is never the same two times
Acknowledgements
I would like to thank cordially K and Dr S Engel for introducing me to the Saiwan region and all their hospitality in Muscat, just as much W Herget, his family, and his colleagues (at this time from Wintershall AG, Muscat) for their warm reception and important assistance Just I would like to thank
Mr K Thomas and his colleagues (at this time from Conquest Oil Exploration Company, Muscat) for their extensive logistical help Furthermore I thank Mrs M Dukat, I Hirsmüller and U Kunz (all TU Darmstadt) for the excellent photographs Special thanks to Dr S Götz (Heidelberg) and Dr J P Platel (BRGM, France) for their time-consuming reviews and the critical and valuable comments The fieldwork was generously supported by the ‘Deutsche Forschungsgemeinschaft’ (DFG), Grant Schu 410/10 – 1, 2, 3
The materials are housed in the ‘Hessisches Landesmuseum Darmstadt’, Section Geosciences, under UVCS (Upper Valve Collection Schumann)
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References
Trang 5PLATE 1
Structures of V vesiculosus
Figure 1. Area of oscule E, particular canalization around the oscule, all canals were roofed by
a sieve, UVCS-180
Figure 2a. Complete canal arrangement See specific arrangement around L, S, E!
Figure 2b. Detail of Figure 2a, oscule and pillar E The surface of the pillar (sp) is crenulated The
steep canals around the oscule are roofed by a sieve The canals end abruptely on the
surface of the pillar, but there is a tiny gap (g) on the canals floor where water could
pass through, UVCS-144 The black line is 5 mm.
Trang 7PLATE 2
Structures of V vesiculosus
Figures 1 , 2 Individual canal arrangement of UVCS-23 and 24 The arrows in Figure 2 show an
area where the sieve and the canals were damaged during life and roughly repaired
Figure 3. A nearly complete roofed canalization, UVCS-32.
Figure 4 Canalization and sieve around S, UVCS-37
Figure 5 Canalization and sieve around E, UVCS-25
Figure 6 Vascular impressions on the margin, lower valve, UVCS-54
Figure 7 Undersurface of upper valve, see structures of the oscules E, S and some of the
main openings (‘gully holes’) at the margin, UVCS-147
Figure 8 Finger-shaped sieve structure on the margin of UVCS-41 The black line is 5 mm.